The invention relates to ink jet printers, particularly to semiconductor chips used for ink ejection and to the structure and construction of the chips which provide reliable, long-life ink jet pens.
Ink jet printers continue to be improved as the technology for making the printheads continues to advance. New techniques are constantly being developed to provide low cost, highly reliable printers which approach the speed and quality of laser printers. An added benefit of ink jet printers is that color images can be produced at a fraction of the cost of laser printers with as good or better quality than laser printers. All of the foregoing benefits exhibited by ink jet printers have also increased the competitiveness of suppliers to provide comparable printers in a more cost efficient manner than their competitors.
One area of improvement in the printers is in the print engine or printhead itself. This seemingly simple device is a microscopic marvel containing electrical circuits, ink passageways and a variety of tiny parts assembled with precision to provide a powerful, yet versatile ink jet pen. The printhead components of the pen must cooperate with each other and with an endless variety of ink formulations to provide the desired print properties. Accordingly, it is important to match the printhead components to the ink and the duty cycle demanded by the printer. Slight variations in production quality can have a tremendous influence on the product yield and resulting printer performance.
The primary components of the ink jet printhead are a semiconductor chip, a nozzle plate and a flexible circuit attached to the chip. The semiconductor chip is typically made of silicon and contains various passivation layers, conductive metal layers, resistive layers, insulative layers and protective layers deposited on a device surface thereof. For thermal ink jet printers, individual heater resistors are defined in the resistive layers and each heater resistor corresponds to a nozzle hole in the nozzle plate for heating and ejecting ink toward a print media. In a top-shooter type printhead, nozzle plates are attached to the chips and there are ink chambers and ink feed channels for directing ink to each of the ejection devices on the semiconductor chip either formed in the nozzle plate material or in a separate thick film layer. In a center feed design for a top-shooter type printhead, ink is supplied to the ink channels and ink chambers from a slot or single ink via which is conventionally formed by chemically etching or grit blasting through the thickness of the semiconductor chip. The chip, nozzle plate and flexible circuit assembly is typically bonded to a thermoplastic body using a heat curable and/or radiation curable adhesive to provide an ink jet pen.
Individual chips are fabricated from a silicon wafer containing many chips. The chips are cut from the wafer during the pen fabrication process and are attached to the pen body. Chips typically measure 2 to 8 mm wide by 10 to 20 mm long by 0.6 to 0.65 mm thick. The chips are delicate and require special care to prevent cracking, breaking or warping during the assembly process.
In order to increase print speed, larger chips are being designed. By increasing the size of the chips, the chips are capable of containing more ink ejectors thereby providing more ink per print swath. However, larger chips also increase the difficulty associated with handling the chips without damage or breakage and larger chips require more care when attaching the chips to a thermoplastic body so as to minimize chip cracking and warpage.
As advances are made in print quality and speed, a need arises for an increased number of ink ejectors which are more closely spaced on the silicon chips. The advances in print speed and quality encourage increases in printhead complexity resulting in a need for long-life printheads which can be produced in high yield while meeting more demanding manufacturing tolerances. Thus, there continues to be a need for improved manufacturing processes and techniques which provide improved printhead components.
With regard to the foregoing, the invention provides an improved ink jet printhead and method for making a printhead for an ink jet pen. The printhead includes a printhead body having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). A semiconductor chip containing ink ejector devices is adhesively attached to the chip surface side of the printhead body. A stiffener is adhesively attached to the ink surface side to provide body stiffening during curing of the adhesive. The semiconductor chip has a second CTE and the stiffener has a third CTE wherein the second and third CTE""s have a similar value.
In another aspect, the invention provides a method for making a printhead for an ink jet printer. The method includes the steps of providing a printhead body having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). An adhesive is applied to the chip surface side of the printhead body. A semiconductor chip containing ink ejector devices and having a second CTE is adhesively attached to the chip surface side of the printhead body using the adhesive. A stiffener having a third CTE is adhesively attached to the ink surface side of the printhead body using the adhesive to provide body stiffening during curing of the adhesive and the adhesive is cured. The second and third CTE""s preferably have a similar value.
In yet another aspect the invention provides an ink jet pen for an ink jet printer. The pen includes an ink container, ink in the ink container and a printhead body attached to the ink container having a chip surface side, an ink surface side opposite the chip surface side and a first coefficient of thermal expansion (CTE). A semiconductor chip containing ink ejector devices and having a second CTE is adhesively attached to the chip surface side of the printhead body. A stiffener having a third CTE is adhesively attached to the ink surface side to provide body stiffening during curing of the adhesive, wherein the second and third CTE""s have a similar value.
An advantage of the invention is that it provides an improved structure for printheads which resist warpage and/or breakage of the semiconductor chips during the manufacturing process used to make the printheads. It has been observed that the chip side of the printhead body is substantially constrained from contracting by the chip and adhesive during the cooling process after curing the chip adhesive, while the unconstrained side of the printhead body is free to expand and contract. This unequal constraint on the printhead body material induces bowing of the printhead body during the curing process sufficient to warp or crack the chip. The invention solves the bowing problem by providing a stiffener on the opposite side of the printhead body from the chip. It is preferred that the stiffener be attached to the printhead body with the same adhesive used to attach the chip and that the stiffener be placed substantially opposite the chip on the opposing surface of the printhead body. Another advantage of the invention is that the printheads exhibit improved impact resistance due to the presence of the stiffener thereby improving product yield and decreasing chip failure during printhead handling in manufacturing or by consumers.